Anwar Ghani

A critique of the microbial metabolic quotient (qCO2) as a bioindicator of disturbance and ecosystem development

Abstract The microbial metabolic quotient (respiration-to-biomass ratio) or qCO2, conceptually based on Odums theory of ecosystem succession, is increasingly being used as an index of ecosystem development (during which it supposedly declines) and disturbance (due to which it supposedly increases). We investigated the suitability of qCO2 as an bioindicator using: (1) data from the Franz Josef Glacier chronosequence, spanning over 22,000 years; and (2) data recalculated from published studies. In the Franz Josef sequence, a detectable decline in qCO2 occurred in the first 250 years in both the L-layer and mineral soil layer. However, in the later phases of the succession there was a sharp increase in qCO2 indicating reduced microbial efficiency, which appeared to be related to stress (independent of disturbance) resulting from steady-state conditions. Calculation of qCO2 from three previous studies on disturbance and ecosystem development indicated that this index responds unpredictably and does not necessarily decline during succession. Plant litter decomposition studies demonstrate that while qCO2 usually declines initially, a significant increase in qCO2 can eventually follow on litter types resistant to decomposition. Correlation analysis of each of 24 previous studies demonstrated that qCO2 often declines with increasing pH, clay content and amounts of microbial biomass; these three soil properties are all indicative of varying stress rather than disturbance levels. Reanalysis of data from 16 previous studies indicated that some disturbances such as fertilization and liming can either increase or decrease qCO2 values depending on whether the disturbance alleviates stress (reducing qCO2)or is more extreme than the stress it alleviates (enhancing qCO2). Although cultivation represents a severe disturbance, qCO2 is not predictably enhanced by this perturbation. While qCO2 undoubtedly provides a useful measure of microbial efficiency, our data suggests it has limitations because it can be insensitive to disturbance and ecosystem development, fails to distinguish between effects of disturbance and stress, and does not decline predictably in response to ecosystem development whenever stress increases along successional gradients.

PLANT REMOVALS IN PERENNIAL GRASSLAND: VEGETATION DYNAMICS, DECOMPOSERS, SOIL BIODIVERSITY, AND ECOSYSTEM PROPERTIES

The consequences of permanent loss of species or species groups from plant communities are poorly understood, although there is increasing evidence that individual species effects are important in modifying ecosystem properties. We conducted a field experiment in a New Zealand perennial grassland ecosystem, creating artificial vegetation gaps and imposing manipulation treatments on the reestablishing vegetation. Treatments consisted of continual removal of different subsets or “functional groups” of the flora. We monitored vegetation and soil biotic and chemical properties over a 3-yr period. Plant competitive effects were clear: removal of the C3 grass Lolium perenne L. enhanced vegetative cover, biomass, and species richness of both the C4 grass and dicotyledonous weed functional groups and had either positive or negative effects on the legume Trifolium repens L., depending on season. Treatments significantly affected total plant cover and biomass; in particular, C4 grass removal reduced total plant biomass in summer, because no other species had appropriate phenology. Removal of C3 grasses reduced total root biomass and drastically enhanced overall shoot-to-root biomass ratios. Aboveground net primary productivity (NPP) was not strongly affected by any treatment, indicating strong compensatory effects between different functional components of the flora. Removing all plants often negatively affected three further trophic levels of the decomposer functional food web: microflora, microbe-feeding nematodes, and predaceous nematodes. However, as long as plants were present, we did not find strong effects of removal treatments, NPP, or plant biomass on these trophic groupings, which instead were most closely related to spatial variation in soil chemical properties across all trophic levels, soil N in particular. Larger decomposer organisms, i.e., Collembola and earthworms, were unresponsive to any factor other than removal of all plants, which reduced their populations. We also considered five functional components of the soil biota at finer taxonomic levels: three decomposer components (microflora, microbe-feeding nematodes, predaceous nematodes) and two herbivore groups (nematodes and arthropods). Taxa within these five groups responded to removal treatments, indicating that plant community composition has multitrophic effects at higher levels of taxonomic resolution. The principal ordination axes summarizing community-level data for different trophic groups in the soil food web were related to each other in several instances, but the plant ordination axes were only significantly related to those of the soil microfloral community. There were time lag effects, with ordination axes of soil-associated herbivorous arthropods and microbial-feeding nematodes being related to ordination axes representing plant community structure at earlier measurement dates. Taxonomic diversity of some soil organism groups was linked to plant removals or to plant diversity. For herbivorous arthropods, removal of C4 grasses enhanced diversity; there were negative correlations between plant and arthropod diversity, presumably because of negative influences of C4 species in the most diverse treatments. There was evidence of lag relationships between diversity of plants and that of the three decomposer groups, indicating multitrophic effects of altering plant diversity. Relatively small effects of plant removal on the decomposer food web were also apparent in soil processes regulated by this food web. Decomposition rates of substrates added to soils showed no relationship with treatment, and rates of CO2 evolution from the soil were only adversely affected when all plants were removed. Few plant functional-group effects on soil nutrient dynamics were identified. Although some treatments affected temporal variability (and thus stability) of soil biotic properties (particularly CO2 release) throughout the experiment, there was no evidence of destabilizing effects of plant removals. Our data provide evidence that permanent exclusion of plant species from the species pool can have important consequences for overall vegetation composition in addition to the direct effects of vegetation removal, and various potential effects on both the above- and belowground subsystems. The nature of many of these effects is driven by which plant species are lost from the system, which depends on the various attributes or traits of these species.

INTRODUCED BROWSING MAMMALS IN NEW ZEALAND NATURAL FORESTS: ABOVEGROUND AND BELOWGROUND CONSEQUENCES

Forest dwelling browsing mammals, notably feral goats and deer, have been introduced to New Zealand over the past 220 years; prior to this such mammals were absent from New Zealand. The New Zealand forested landscape, therefore, presents an almost unique opportunity to determine the impacts of introduction of an entire functional group of alien animals to a habitat from which that group was previously absent. We sampled 30 long-term fenced exclosure plots in indigenous forests throughout New Zealand to evaluate community- and ecosystem-level impacts of introduced browsing mammals, emphasizing the decomposer subsystem. Browsing mammals often significantly altered plant community composition, reducing palatable broad-leaved species and promoting other less palatable types. Vegetation density in the browse layer was also usually reduced. Although there were some small but statis- tically significant effects of browsing on some measures of soil quality across the 30 locations, there were no consistent effects on components of the soil microfood web (com- prising microflora and nematodes, and spanning three consumer trophic levels); while there were clear multitrophic effects of browsing on this food web for several locations, com- parable numbers of locations showed stimulation and inhibition of biomasses or populations of food web components. In contrast, all microarthropod and macrofaunal groups were consistently adversely affected by browsing, irrespective of trophic position. Across the 30 locations, the magnitude of response of the dominant soil biotic groups to browsing mammals (and hence their resistance to browsers) was not correlated with the magnitude of vegetation response to browsing but was often strongly related to a range of other variables, including macroclimatic, soil nutrient, and tree stand properties. There were often strong significant effects of browsing mammals on species composition of the plant community, species composition of leaf litter in the litter layer, and composition of various litter-dwelling faunal groups. Across the 30 locations, the magnitude of browsing mammal effects on faunal community composition was often correlated with browser effects on litter layer leaf species composition but never with browser effects on plant community composition. Browsing mammals usually reduced browse layer plant diversity and often also altered habitat diversity in the litter layer and diversity of various soil faunal groups. Across the 30 locations, the magnitude of browser effects on diversity of only one faunal group, humus-dwelling nematodes, was correlated with browser effects on plant diversity. However, browser effects on diversity of diplopods and gastropods were correlated with browser effects on habitat diversity of the litter layer. Reasons for the lack of unidirectional relationships across locations between effects of browsers on vegetation community attri- butes and on soil invertebrate community attributes are discussed. Browsing mammals generally did not have strong effects on C mineralization but did significantly influence soil C and N storage on an areal basis for several locations. However the direction of these effects was idiosyncratic and presumably reflects different mechanisms by which browsers affect soil processes. While our study did not support hypotheses predicting consistent negative effects of browsing mammals on the decomposer subsystem through promotion of plant species with poorer litter quality, our results still show that the introduction of these mammals to New Zealand has caused far-ranging effects at both the community and ecosystem levels of resolution, with particularly adverse effects for indigenous plant com- munities and populations of most groups of litter-dwelling mesofauna and macrofauna.

Why is the strength of relationships between pairs of methods for estimating soil microbial biomass often so variable

Abstract Physiological and biochemical methods for estimating soil microbial biomass are usually calibrated against other methods and parameters. However, while calibrations are usually made over soils with a very wide range of microbial biomass values (across a wide geographical range) they are often used to assess relatively small differences in microbial biomass across a narrower range of microbial biomass values (across a smaller geographical area, e.g. within a single field), where their reliability may be considerably less. We investigated the abilities of three methods of quantifying microbial biomass, i.e. substrate-induced respiration (SIR), fumigation-incubation (FI) and fumigation-extraction (FE) to serve as predictors of each other across two geographical scales, i.e. across 12 sites over an area of ≈ 100 × 100 km; and within each of these sites (12 samples per site) over an area of 0.3 ha each. Over the larger scale, relationships between pairs of methods were strong, with R 2 values always > 0.90. However, over the smaller scale, correlations between pairs of methods were variable and only significant for those sites in which spatial variability in soil organic matter was relatively high. Uncertain relationships between SIR and the fumigation-based methods may be expected because they apply to different subsets of the soil biomass (i.e. glucose-responsive vs chloroform-sensitive). However, we suggest that FI and FE are sometimes weakly correlated because the FI decomposition constant k c and the FE constant k EC vary differently relative to each other across underlying gradients. Calibration equations for estimating microbial biomass are most accurate when restricted to situations where the range of biomass values is comparable to that from which the calibration was first derived, and to similar soil types. In our study, it appears that calibration equations for predicting microbial biomass are only likely to provide reliable relative estimates in situations where the coefficient of variation (standard deviation/mean) of soil organic C is > 15%.

Enhancement of phosphate rock solubility through biological processes

Abstract Biological acidulation of a reactive phosphate rock (PR), Sechura, was investigated by exposing ground PR to various combinations of elemental sulphur, soil suspension enriched with Thiobacilli, nutrient solution and lactic casein whey. The incubation was carried out at 30°C for 4 weeks. Weekly measurements were made of the pH of the mixture, phosphorus extractable with water (water-P) and 0.5 M NaHCO, (bicarb-P) solution, biomass-P, residual elemental sulphur and the Thiobacillus population. Treatments without elemental S or Thiobacilli culture had little effect on the dissolution of PR. The combination of elemental S and Thiobacilli increased extractable P to nine times that of the untreated PR and with the addition of nutrient solution this was enhanced to 15 times. This was equivalent to 9–10% of stoichiometric acidulation of the PR with sulphuric acid. However, the amount of acid produced, calculated from the sulphur oxidised, was equivalent to that of 14% acidulation. Use of lactic whey with PR alone or in combination with elemental S showed either an immobilisation of soluble P or very little increase in water-P compared with control values. Amounts of P dissolved during incubation which might have been immobilised were determined by the fumigation-incubation technique for recovering P in the microbial biomass. This was found to be a very small fraction of the total P dissolved during the incubation.

Fate of 14C from glucose and the herbicide metsulfuron–methyl in a plant–soil microcosm system

Abstract The effects of weeds and soil nutrient additions on the fate of 14 C-labelled glucose and metsulfuron–methyl (metsulfuron) herbicide were determined in a growth-chamber soil–plant microcosm study. Nodding thistle ( Carduus nutans ) was used as a test weed species to which these compounds were applied. The experiment consisted of a full factorial design of N addition, P addition, planting with C. nutans , and addition of 14 C-labelled compounds (glucose and herbicide metsulfuron). The experiment ran for 131 days following addition of the 14 C-labelled compounds. There was a significant positive effect of C. nutans plants on soil microbial biomass-C and basal respiration. The addition of nutrients did not consistently affect any of the parameters measured in this study. The fate of 14 C-labelled compounds was determined by quantifying the relative amounts of 14 C absorbed by plants, incorporated into soil, taken up into microbial biomass and respired by soil organisms. Two days after application of the 14 C compounds, 7–8% of the 14 C was absorbed by the C. nutans (weed) canopy. By day 131, approximately 2–3% of the applied 14 C was still detected in the tissues of dead thistle plants. The amounts and pattern of decomposition differed considerably between metsulfuron and glucose. Almost all (99%) of the applied 14 C glucose was either respired by the microbes or incorporated into microbial biomass by day 131 while only 38–42% of the applied metsulfuron was respired or incorporated into microbes by this time. Greater amounts of metsulfuron pesticide were degraded in microcosms containing C. nutans plants than in those without plants. By day 131, 27.4% of 14 C added from metsulfuron was respired in the planted treatment and only 22.8% from the non-planted treatment. This would suggest that the presence of decomposing weed litter (dying weed plants) has the potential to enhance the decomposition of added xenobiotics such as herbicides.

Biodiversity of indigenous tussock grassland sites in Otago, Canterbury and the central North Island of New Zealand III. Soil microorganisms

Abstract Bacterial and fungal communities in indigenous tussock grassland soils were studied at four locations, Mt Benger and Deep Stream (Otago), Cass (Canterbury) and Tukino (central North Island). Soil samples collected from inter‐tussock and tussock areas were used to enumerate total culturable bacteria (colony forming units (CFU)) as well as a number of specific groups of bacteria, fungal types and diversity, and soil microbial functional diversity. Soil microbial biomass carbon (C) and nitrogen (N) as well as extractable C and N were also determined. Fungal populations were lowest at Mt Benger while fluorescent Pseudomonas was lowest at Cass. In these indigenous soils, bacterial CFU were c. 30 times lower and fungal populations c. 10 times greater than in developed New Zealand pastoral soils. CFU of bacteria in complex media (r‐strategists) were similar in magnitude to bacteria growing on weak CA medium (K‐strategists). The microbial biomass C in indigenous grassland soils were also significantly greater than that found in developed pastoral soils, possibly due to dominance of fungi in indigenous soils. Microbial C and N were lower in Cass and Tukino sites compared to Mt Benger and Deep Stream sites. Microbial nitrogen was also significantly greater (P < 0.001) in inter‐tussock than under tussock samples. The functional diversity of soil microbes, was significantly greater (P< 0.01) in inter‐tussock samples compared to tussock samples at all sites except Mt Benger. The total microbial activity, as measured by the colour development in Biolog plates (Average Well Colour Development (AWCD)), was low in Cass soil compared to others. Eighty‐three types of fungi belonging to 30 genera were recovered from the four sites. The diversity of fungi found in Otago sites was greater than in the other two sites (P < 0.001). Fusarium spp. that are often common in pastoral soils were conspicuous by their rarity in indigenous soils.

Differential influence of soil pH on the availability of partially sulphuric and phosphoric acidulated phosphate rocks II. Chemical and scanning elecronmicroscopic studies

Part I of this study showed that the plant availability of P from a reactive phosphate rock (PR), North Carolina PR, partially acidulated with phosphoric acid (Phos-PAPR) increased with decreasing soil pH from pH 6.1 to 5.1, whereas availability from a blend of similarly reactive PRs partially acidulated with sulphuric acid (SA-PAPR) changed little. The present study was carried out to explain the above results. Phosphate sorption maximum of soil as a function of soil pH was determined. Soil samples obtained at the completion of the pot experiment [5] were analysed for inorganic P fractions, and the amounts of PR dissolved from the PAPRs were determined. A leaching study, simulating pot experiment conditions, was conducted to determine the changes in the chemical composition and the spatial distribution of P, S and Ca in the fertilizer residues. The properties of the PAPRs were further characterised by sequential extraction of the fertilizers.Phosphate sorption isotherms indicated a smaller amount of P in solution at lower pH values, which suggested reduced P availability with decreasing soil pH. Dissolution of the residual PR-P was generally greater in Phos-PAPR treatment than in PR applied directly or in the SA-PAPR treatment. PR-P dissolution in Phos-PAPR increased with decreasing pH but not in SA-PAPR. Chemical, electron microprobe, X-ray micro-analysis and X-ray powder diffraction studies of the fertilizer residues obtained from the leaching and sequential extraction experiments showed rapid dissolution of the Ca(H2PO4)2 phase of the fertilizers but the CaSO4.XH2O persisting as a cementing phase between the PR particles. The CaSO4.XH2O which intially existed mostly in an anhydrous form changed to gypsum. It was concluded that the dissolution of PR-P in the SA-PAPR was impeded by the presence of CaSO4.XH2O acting as a physical barrier and also by providing higher Ca in solution than that would exist in a saturated solution of the apatites.

Effects of dairy factory effluent application on nutrient transformation in soil

Abstract Dairy factory effluent (DFE) contains significant amounts of nutrients such as nitrogen (N), phosphorus (P), potassium (K), and sulphur (S) which are beneficial to plant growth. It also contains high amounts of carbon (C). Lately, there has been some concern that DFE application to pastoral land is adversely affecting plant growth in some regions of New Zealand. In this study, we determined the mineralisation and immobilisation of nutrients particularly C, N, S, and cations, in a DFE‐treated Omeheu sandy loam soil. We report findings from laboratory‐based open incubation studies carried out at 10, 20, and 30°C, with four rates of DFE application (0, 150 000, 300 000, and 450 000 litres ha–1) alone and with added NO3 – (100 kg N ha–1). The DFE was applied at two‐weekly intervals into packed soil columns which were leached with 0.01 MCaCl2 solution. Leachates were analysed for total C, total N, SO4 2–, NO3 ‐, NH4 +, K+, Na+, and Mg2+. Effects of DFE application on soil microbial bio‐mass‐C, hot‐water extractable‐C, and anaerobically mineralisable‐N were also determined. Addition of DFE increased the size of the microbial biomass pool and thereby enhanced immobilisation of nutrients, mainly N and S. The immobilisation was greater at higher temperature. At 10°C, microbes were unable to utilise all of the added C, even at the lowest rate of DFE application, and 40–50% of the C was leached from soil columns. However, at 30°C soil microbes either immobilised or respired between 95–97% of the C added from DFE, and only small amounts of C were measured in the leachates. Addition of NO3 –‐N had no significant influence on the C immobilisation or respiration. Most of the added N (92–97%) from DFE remained immobilised in the soils throughout the study. A high proportion of the NO3 –‐N added with DFE was immobilised in soils at 10 and 20°C, showing the dominating influence of soluble C, added through the two‐weekly application of DFE, in stimulating microbial activity and causing a prolonged immobilisation of N. There was a net mineralisation of about 100 μg NO3 –‐N g–1 soil at 30°C, indicating faster metabolic use of soluble C from DFE by microbes at this temperature. Between 15–35% of the SO4 2–‐S applied from DFE was either immobilised by soil microbes or was adsorbed on soil organic matter. The presence of significant amounts of NH4 + in DFE‐treated soils suggests that parts of the soil columns may have become anaerobic during incubation, causing mineralisation of N from the death of aerobic microbes or decomposition of soil organic matter. A high proportion of the cations (K+, Na+, and Mg2+) that were added with DFE leached out, indicating that DFE application would have very little effect on the availability of these cations for plant uptake. This study, in part, explains that the poor performance of DFE application on pastoral soils predominantly arises through its effects on the availability of N for plant growth.

Improving ryegrass-clover pasture dry matter yield and urea efficiency with gibberellic acid

BACKGROUND The effects of spraying gibberellic acid (GA3) at 20 or 30 g ha(-1), with or without application of urea, on pasture dry matter (DM) yield, herbage nitrogen (N) concentration and feed quality were investigated in 2011 and 2012 for managed pastoral systems in New Zealand across a range of sites, in both autumn and spring. RESULTS On the Waikato site (autumn and spring, 2012), and at all five sites in 2011, liquid urea applied with GA3 at 20 or 30 g ha(-1) consistently produced significantly higher pasture shoot DM yield, relative to liquid urea alone. Application of GA3 alone reduced feed quality by lowering metabolizable energy, crude protein and organic matter digestibility values. However, a reduced feed quality was not observed when GA3 was applied together with liquid urea. Liquid urea applied with GA3 also reduced total N and nitrate-N concentration in herbage, relative to liquid urea applied alone. CONCLUSION Application of GA3 together with liquid urea provides an opportunity for the strategic use of urea to meet both production and environmental goals.